Diesel or lean burn gasoline vehicles of high energy efficiency emit less carbon monoxide (CO) and hydrocarbon (HC) in their exhaust gas, but the emission of nitrogen oxides (NOx) thereof shows a tendency to increase (J.-H. Park, S. J Park, I.-S Nam, Catal. Surv. Asia. 14 (2010) 11).
Among the nitrogen oxides, nitrogen monoxide (NO) causes a respiratory disease and contributes to global warming and acid rain. Accordingly, efforts to reduce the nitrogen oxides in exhaust gas emitted from cars have been ongoing, and representatively Urea-SCR and LNT catalyst technologies are known in the art. They are generally located at the back end of DOC and are designed to maximize the decrease of the nitrogen oxides in the Urea-SCR and LNT reactor through the oxidation reaction of CO, HC and NO in the exhaust gas (L. Olsson, H. Karlsson, Catal. Today, 147S (2009) S290).
Generally, it is known that a noble metal catalyst, such as platinum, has been used as the NO oxidation catalyst. However, due to the rapid increase of the price of noble metals in recent years, the preparation cost of platinum-based catalysts has increased largely. In addition, the platinum-based catalysts have a problem of catalyst activity decrease due to a deterioration phenomenon according to long-term use.
Meanwhile, through Korean Patent Publication No. 10-2013-0137868, the present inventors have recognized the fact that the perovskite catalyst having lanthanum, manganese and silver as the elements shows an excellent ability as the oxidation catalyst for nitrogen monoxide.
The preparation method of perovskite catalyst disclosed in said Patent Publication is a citric acid method, and the preparation processes are as follows. First, a metal precursor solution was prepared by dissolving La(NO3)3.6H2O 10.5597 g, Mn(NO3)2.6H2O 14.0000 g and AgNO3 4.1426 g in a distilled water. An excess of citric acid 10 wt % more than the amount corresponding to the mole of the metal precursors was added to the solution and the solution was stirred for 1 hour. Subsequently, the solution was heated to 80° C. and the distilled water was slowly evaporated by maintaining the temperature for 8 hours with continuously stirring. The perovskite catalyst of gel phase from which over certain amount of the distilled water was eliminated was completely dried overnight in a 110° C. oven. Finally, perovskite catalyst of La0.5Ag0.5MnO3 was synthesized through pre-combustion (250° C.) and calcination (700° C.) under atmospheric conditions for 5 hours.
However, in the preparation method of perovskite disclosed in said Patent Publication, the collect rate of the residue in the crucible for the catalyst was less than 10% because the raw materials were exploded and scattered during the preparation process. Therefore, the preparation method has disadvantages in that it must be carried out in a sealed place, the collect rate of the catalyst is low, and the product is easily contaminated because most of the prepared catalyst exists on the floor.
For this reason, the present inventors have conducted a study on a stable preparation method of perovskite catalyst that is very effective as an oxidation catalyst for nitrogen oxides, particularly, nitrogen monoxide, and completed the present disclosure by recognizing the fact that it is possible to resolve the problem by applying a low temperature dry condition to the method as disclosed below.